pghoard: added a thread for processing the "download_results" queue

Added a new DownloadResultsProcessor thread, which validates downloaded files and saves them to the target: '<pg_wal>/<wal_name>.tmp' -> '<pg_wal>/<wal_name>.prefetch'.
Files with "prefetch" suffix can be copied to the destination without extra checks now.
Also changed items type for "pending_download_ops" dict from "dict" to "dataclass" (PendingDownloadOp)

pghoard/common.py

@@ -448,8 +448,8 @@ class UnhandledThreadException(Exception):
 
 
 class PGHoardThread(Thread):
-    def __init__(self):
-        super().__init__()
+    def __init__(self, name: Optional[str] = None):
+        super().__init__(name=name)
         self.exception: Optional[Exception] = None
 
     def run_safe(self):